ARM: ux500: add board documentation

This documents the device tree bindings on the top level of
the Ux500 boards.
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/arm/ux500/boards.txt

+ST-Ericsson Ux500 boards
+------------------------
+
+Required properties (in root node) one of these:
+	compatible = "st-ericsson,mop500" (legacy)
+	compatible = "st-ericsson,u8500"
+
+Required node (under root node):
+
+soc: represents the system-on-chip and contains the chip
+peripherals
+
+Required property of soc node, one of these:
+	compatible = "stericsson,db8500"
+
+Required subnodes under soc node:
+
+backupram: (used for CPU spin tables and for storing data
+during retention, system won't boot without this):
+	compatible = "ste,dbx500-backupram"
+
+scu:
+	see binding for arm/scu.txt
+
+interrupt-controller:
+	see binding for arm/gic.txt
+
+timer:
+	see binding for arm/twd.txt
+
+clocks:
+	see binding for clocks/ux500.txt
+
+Example:
+
+/dts-v1/;
+
+/ {
+        model = "ST-Ericsson HREF (pre-v60) and ST UIB";
+        compatible = "st-ericsson,mop500", "st-ericsson,u8500";
+
+        soc {
+		#address-cells = <1>;
+		#size-cells = <1>;
+		compatible = "stericsson,db8500";
+		interrupt-parent = <&intc>;
+		ranges;
+
+		backupram@80150000 {
+			compatible = "ste,dbx500-backupram";
+			reg = <0x80150000 0x2000>;
+		};
+
+		intc: interrupt-controller@a0411000 {
+			compatible = "arm,cortex-a9-gic";
+			#interrupt-cells = <3>;
+			#address-cells = <1>;
+			interrupt-controller;
+			reg = <0xa0411000 0x1000>,
+			      <0xa0410100 0x100>;
+		};
+
+		scu@a04100000 {
+			compatible = "arm,cortex-a9-scu";
+			reg = <0xa0410000 0x100>;
+		};
+
+		timer@a0410600 {
+			compatible = "arm,cortex-a9-twd-timer";
+			reg = <0xa0410600 0x20>;
+			interrupts = <1 13 0x304>; /* IRQ level high per-CPU */
+			clocks = <&smp_twd_clk>;
+		};
+
+		clocks {
+			compatible = "stericsson,u8500-clks";
+
+			smp_twd_clk: smp-twd-clock {
+				#clock-cells = <0>;
+			};
+		};
+        };
+};